Films composed principally of a PAS typified by poly(phenylene sulfide) which may hereinafter be called "PPS" have excellent properties such as high heat resistance, chemical resistance and mechanical strength, and good electrical properties and are useful for various industrial applications.
Stretched PAS films however involve a problem that they tend to undergo deformations such as heat shrinkage in a high temperature range. Unstretched PAS sheets of smaller orientation are hence used for applications where high-temperature dimensional stability is required. In this case, since PAS has a lower glass transition temperature, its crystallization is indispensable in order to impart heat resistance to the sheets. The reason is that among conventional unstretched PAS sheets, uncrystallized ones show significantly lowered strength when the temperature increases to 100.degree. C. or so and they hence undergo great deformation under a slight load. As illustrated in FIG. 1 by way of example, when the unstretched sheets are heated up at a constant rate under a slight fixed load, an uncrystallized sheet shows a high elongation from about 120.degree. C. as depicted by a line (3), indicating that it is deformed to a great extent by the slight load. On the other hand, a sheet having a degree of crystallization of 6% does not exhibit such a great deformation at about 120.degree. C. as the uncrystallized sheet has shown as depicted by a line (1). Further, it is found that a sheet crystallized to a degree of crystallization sufficiently as high as 24% shows a little deformation at high temperatures as depicted by a line (2) and its high-temperature dimensional stability is hence good. Unstretched PAS sheets sufficiently crystallized by conventional crystallization processes are however accompanied by disadvantages that they have a low elongation and are liable to be brittle.
With respect to the improvements of unstretched PPS sheets, various proposals have been made in, for example, Japanese Patent Publication No. 42611/1984, and Japanese Patent Application Laid-Open Nos. 121052/1982, 184619/1984 and 251121/1987 to date.
Conventionally, crystallization of an unstretched PAS film has been effected by subjecting an amorphous sheet to a heat treatment in a temperature range of from the glass transition point of PAS or higher to its melting points or lower. Namely, a sheet-like formed product composed principally of PPS has generally been produced by melting the starting resin, extruding the melt through a slit die, cooling and solidifying the extrudate into an amorphous sheet, and then subjecting the sheet to a heat treatment.
The conventional heat treatments for crystallization include, for example, a method in which a sheet to be treated is brought into contact with a heated liquid or gas stream or a surface of a heated solid such as a roll (Japanese Patent Publication No. 42611/1984). It has also been known to smoothen the surface of a sheet-like material by subjecting the sheet-like material to a heat treatment while supporting it with clamps or the like at its periphery or causing it to continuously pass through a hot-air oven in a state supported at one or two points continuously or heat treating it on a smooth stainless steel belt, followed by compression forming or pressing between pressure rolls (Japanese Patent Application Laid-Open No. 184619/1984).
These conventional heat treatment methods are however difficult to provide sheets excellent in both planarity and smoothness when unstretched PAS sheets are industrially produced. Moreover, the provision of a smooth PAS sheet requires a complex step such as compression forming or rolling, so that larger production facilities are required.
Incidentally, the behavior of a PAS sheet upon its heat treatment includes that a sheet cooled and solidified in an amorphous state is exposed to a temperature above the glass transition point owing to its heating and upon a lapse of a predetermined time, is crystallized and hardened. When a PAS sheet is subjected to a heat treatment in a heated liquid or gas stream by way of example, the sheet expands and becomes sticky as the temperature increases. When the temperature increases beyond the glass transition point of the PAS and the sheet becomes soft, the sheet is distorted or locally elongated, sticks to another material or object which is in contact with the sheet, or forms a roughened surface due to eruption of low boiling materials contained inside the PAS. Crystallization thereafter proceeds, and the sheet shrinks volumetrically by its density increment accompanied by the crystallization and hence undergoes changes in dimension corresponding to the volumetric shrinkage, thereby hardening the sheet. The resultant sheet is however poor in planarity and its surface conditions are inferior.
In addition, in the crystallization owing to heated air, a PAS becomes very soft at its glass transition point or higher and a PAS sheet deforms and/or breaks due to a slight wind pressure. Indeed, it is hence extremely difficult to obtain a sheet excellent in smoothness. Further, the growth of spherulites is also remarkable. It is hence only possible to obtain a sheet inferior also in planarity.
As described above, the PAS sheet expands with heat and becomes soft in the course of the heat treatment. Therefore, unless the sheet is mechanically fixed during the heat treatment, the planarity of the sheet is reduced and thickness irregularity occurs, thereby deteriorating its appearance.
In the heat treatment method in which a PAS sheet is simply brought into contact with a surface of a solid such as a heating roll or stainless steel belt, the sheet expands and moreover becomes sticky as the temperature increases and at the same time, volumetric shrinkage caused by crystallization occurs. Accordingly, the sheet may locally and slightly float from the surface of the solid. In addition, entrainment of air is also observed. Subsequent crystallization results in hardening of the sheet. In this case, height differences arise in the surface of the sheet between areas maintain in contact with the solid and those floated from the solid. It is hence only possible to obtain a sheet inferior in planarity.
When a PAS sheet is subjected to a heat treatment by means of a tenter while holding it with clips or the like, the clipped parts become useless and moreover, the resultant sheet is susceptible to breakage from the clipped parts. Besides, the tenter involves an economical problem because its equipment cost and operating cost are expensive.
Even if such a crystallized PAS sheet of poor planarity is pressed by compression forming or rolling, it is impossible to fully remove the thickness irregularity, warpage, small ruggedness and the like to make the sheet excellent in planarity and smoothness because it has already been crystallized. Moreover, the process is complex and there is hence a disadvantage also from economical consideration.
The present inventors previously found that a PAS sheet excellent in planarity and smoothness can be obtained by upon heat treatment of an amorphous PAS sheet through a heating roll, preheating the sheet the then causing the thus-preheated sheet to pass between the heating roll and a pinch roll under a pinch pressure of 0.05-10 kg/cm, whereby the sheet is continuously pressed under linear pressure, and applied for a patent (Japanese Patent Application No. 329542/1987). As has been described therein, when the PAS sheet is crystallized on the heating roll while controlling temperature and contact pressure by making use of the pinch roll, its planarity can be improved to a great extent compared to conventional sheets. There are however potential problems that the ruggedness on surface of the pinch roll is transferred on the surface of the sheet and/or coarse spherulites generate. Therefore, this process is still insufficient to use in fields where high planarity and smoothness are required. In addition, a separate step for conducting the heat treatment is required in this process.
It has been proposed to in the production of a crystallized polyether ether ketone film, conduct cooling and crystallization of film at one stage by controlling the temperature of a casting roll to a temperature in a range of 150-250.degree. C. (Japanese Patent Application Laid-Open No. 92430/1988). When this process is applied to the production process of a sheet making use of a conventional PAS low in melt crystallization temperature, Tc.sub.2, crystallization of the sheet on the casting roll is insufficient, and moreover since the PAS is characterized by high susceptibility to elongation when it is in the amorphous state at a temperature not lower than its glass transition point, the sheet adheres closely to the roll and hence becomes poor in separation property from the roll. It is hence only possible to obtain a sheet inferior in planarity, smoothness and physical properties.
Moreover, when the PAS is used, there is a potential case where a resulting sheet may have low flexing properties even if the sheet is good in appearance. It is hence necessary to select suitably the temperature and time ranges of the process.